The present invention relates to a chain tensioner for use in a chain system for driving camshafts of a vehicle engine, particularly a vehicle engine that rotates at a high speed such as a two-wheeled vehicle engine.
A chain tensioner is used in a chain system for driving camshafts of a vehicle engine, and typically comprises a housing formed with a cylinder chamber, a plunger slidably mounted in the cylinder chamber of the housing, thereby defining a pressure chamber in the cylinder chamber, and a spring mounted in the pressure chamber to bias the plunger outwardly of the cylinder chamber. The housing is formed with a hydraulic oil supply passage communicating with the pressure chamber. The plunger has its protruding end pressed against a camshaft-driving chain. Hydraulic oil in the pressure chamber dampens the pushing force applied from the chain to the plunger, thereby keeping the tension in the chain at a constant level. As the hydraulic oil, engine oil may be supplied into the tensioner.
The applicant has proposed in JP patent publication 2004-176821 a chain tensioner including a relief valve for discharging hydraulic oil in the tensioner if the internal hydraulic pressure exceeds a predetermined level, thereby stabilizing the damping function of the tensioner. The relief valve includes a valve hole and a discharge hole, and is mounted in a stepped hole formed in an oil discharge passage communicating with the pressure chamber.
JP patent publication 2001-32896 discloses a tensioner for a camshaft driving chain or timing belt for use in a two-wheeled or four-wheeled vehicle. This tensioner includes a first shaft and a second shaft having external and internal threads, respectively, that are in threaded engagement with each other and slidably supported on bearings. The spring rotates the first shaft. When the first shaft rotates, the second shaft moves axially, thus applying tension to the chain or belt. A film comprising a solid lubricant is formed between the internal and external threads, and between the first and second shafts and the respective bearings so as to reduce frictional resistance between the first and second shafts and between the first and second shafts and the respective bearings.
Today's engines for two-wheeled vehicles are rotated at higher speed and produce higher horsepower. As a result, conventional chain tensioners cannot sufficiently suppress high-frequency vibrations of the chain used in such an engine, and/or cannot maintain the tension in the chain to an optimum level in a wide revolving speed range of the engine. Various improvements have been proposed including those disclosed in the above-referenced patent publications. But even such improved chain tensioners cannot still maintain the tension in the chain used in a two-wheeled vehicle engine to an optimum level in a wide temperature range and a wide engine revolution range because the revolution range of a two-wheeled vehicle engine is wide.
For example, if the damping force is determined at a low level while the engine is rotating at a high speed in order to reduce the tension in the chain, the damping force may be insufficient while the hydraulic oil temperature is high due to low viscosity of the oil. Conversely, if the damping force is set at a high level, the damping force may be too large when the oil temperature is low and thus high in viscosity. In JP patent publication 2004-176821, if the internal pressure exceeds a predetermined value, the relief valve opens, thus discharging hydraulic oil in the pressure chamber. The chain tensioner can thus stably perform the damping function. But the relief valve alone cannot sufficiently control the tension in the chain used for a two-wheeled vehicle engine.
This is because a two-wheeled vehicle engine has a wide engine revolution range, so that it is necessary to control the tension in the belt even after the engine revolution has exceeded a point at which the tension in the chain is maximum. For this purpose, it is necessary to keep the reaction force produced in the chain tensioner from rising after the relief valve has opened. But if the vibration frequency is high after the relief valve has opened, the pressure in the pressure chamber may increase in spite of the fact that hydraulic oil is being discharged through the relief valve. In order to prevent such a pressure rise after the relief valve has opened, it would be possible to increase the flow rate of hydraulic oil discharged through the relief valve and/or to reduce the pressure at which the relief valve opens. But such a solution may cause a sharp drop in the internal pressure immediately after the relief valve has opened especially if the oil temperature is high and thus its viscosity is low. This will cause flapping of the chain.
In JP patent publication 2001-32896, the frictional resistance produced at the metallic slide contact portions serves as a damper. It is difficult to stably keep such frictional resistance. In order to prevent such frictional resistance from decreasing due to deterioration at the slide contact portions, a film of a solid lubricant is formed at the slide contact portions. This solution is however expensive, and still such a film cannot stabilize the operation of the chain tensioner for a sufficiently long period of time if such a tensioner is used for a camshaft driving chain of a two-wheeled vehicle engine.
An object of the present invention is to provide a chain tensioner especially suitable for use in a chain system for driving cams of a two-wheeled vehicle engine, which can keep the tension in the chain to such a level that the vibration of the chain is minimum at any temperature of the hydraulic oil and at any revolution speed of the engine.